This invention relates to a method of and apparatus for repelling aquatic creatures such as elasmobranchs and sharks in particular.
The invention finds particular application in the protection of surfers (or surfboards) and personal watercraft such as surf skis, paddle skis and the like from shark attack and it will be described with reference to a surfboard application by way of non-limiting example.
U.S. Pat. Nos. 3,686,280 (Holt) and 3,164,172 (Hicks), describe shark repelling devices utilising pulse generators producing an electric field to divert sharks from the proximity of is the generating apparatus. These early devices are referred to in U.S. Pat. Nos. 3,822,403 (Hicks), 4,667,431 (Mendicino) and 4,211,980 (Stowell).
The patent to Mendicino describes a device similar to a cattle prod or human crowd control tazor, but unlike these devices which are designed for mammals and which operate on high voltages (up to 40,000V) and amperages in the milliamp range, the device described by Mendicino provides a 1-5 Amp, 300V-1000V charge in an attempt to repel sharks.
The patent to Stowell describes a method for repelling sharks by creating, about an electrode submerged in salt water, an electric field with a voltage gradient of sufficient magnitude to “overstimulate” (according to the patent) the nervous system of the shark. He describes a system which applies brief DC pulses to electrodes immersed in salt water with a relatively long delay between pulses (0.5 to 10 ms pulses spaced to a repetition rate of between 6 and 12 Hz).
The patent to Hicks describes the use of current pulses to electrodes to create an electric field between the electrodes at a low frequency of approximately 70 cycles per minute.
U.S. Pat. No. 5,566,643—Charter (Natal Sharks Board) is based on the observation that devices that utilise unidirectional current flows, such as the devices described above, tend to develop a positively charged region about the device cathode. Referring to the research of Dr. Kalmijn and others (Dr. Adrianus J Kalmijn—Woods Hole Oceanographic Institution and the US Office of Naval Research) which tends to show that such a positively charged region serves as an attractant to sharks, the Charter patent describes a device that alternates the current flow between electrodes.